In cold climates, there has long been a problem with the freezing of sewer vent pipes. These pipes extend upwardly through a roof and tend to freeze over at the upward end thereof, thereby causing sewage gas to back up in the house, to the detriment of the occupants thereof. In the past, pipe heaters of various sorts have been well-known for assisting in defrosting. Such heaters may take the form of tapes or the like, but due to the nature and the amount of heat involved are generally limited to the metallic pipes. At least one plastic pipe heater is known as typified in U.S. Pat. No. 4,110,603, which shows the heating element spaced slightly from the plastic pipe for defrosting purposes. However, none of these prior defrosting devices are truly suitable for a plastic pipe which generally extends upwardly from a roof line.